[PATCH] Fix misrouted interrupts deadlocks

While testing kernel on machine with "irqpoll" option I've caught such a
lockup:

__do_IRQ()
spin_lock(&desc->lock);
desc->chip->ack(); /* IRQ is ACKed */
note_interrupt()
misrouted_irq()
handle_IRQ_event()
if (...)
local_irq_enable_in_hardirq();
/* interrupts are enabled from now */
...
__do_IRQ() /* same IRQ we've started from */
spin_lock(&desc->lock); /* LOCKUP */

Looking at misrouted_irq() code I've found that a potential deadlock like
this can also take place:

1CPU:
__do_IRQ()
spin_lock(&desc->lock); /* irq = A */
misrouted_irq()
for (i = 1; i < NR_IRQS; i++) {
spin_lock(&desc->lock); /* irq = B */
if (desc->status & IRQ_INPROGRESS) {

2CPU:
__do_IRQ()
spin_lock(&desc->lock); /* irq = B */
misrouted_irq()
for (i = 1; i < NR_IRQS; i++) {
spin_lock(&desc->lock); /* irq = A */
if (desc->status & IRQ_INPROGRESS) {

As the second lock on both CPUs is taken before checking that this irq is
being handled in another processor this may cause a deadlock. This issue
is only theoretical.

I propose the attached patch to fix booth problems: when trying to handle
misrouted IRQ active desc->lock may be unlocked.

Acked-by: Ingo Molnar <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/irq/spurious.c

···
 	if (unlikely(irqfixup)) {
 		/* Don't punish working computers */
 		if ((irqfixup == 2 && irq == 0) || action_ret == IRQ_NONE) {
-			int ok = misrouted_irq(irq);
+			int ok;
+
+			spin_unlock(&desc->lock);
+			ok = misrouted_irq(irq);
+			spin_lock(&desc->lock);
 			if (action_ret == IRQ_NONE)
 				desc->irqs_unhandled -= ok;
 		}